One of many conventional .lambda./4 shifted distributed feedback laser diodes is described in the reports entitled "Characteristics of Quarter-Wave Shifted DFB Laser Diode (No. 1017)" and "Fabrication of 1.5 .mu.m Phase-Shifted DFB Lasers (No. 1018)" in "The Institute of Electronics and Communication Engineers of Japan" held on Mar. 26 to 28, 1984. The distributed feedback laser diode (simply called "DFB LD" hereinafter) comprises a buffer layer on a substrate, an active layer on the buffer layer, a cladding layer on the active layer, a cap layer on the cladding layer, a SiO.sub.2 stripe on the cap layer, and anode and cathode electrodes respectively provided on the outer surfaces of the SiO.sub.2 stripe and substrate wherein a diffraction grating having a .lambda./4 shift is formed on the substrate, and an cleaved facet is provided on an output side of the DFB LD.
In operation, a pulse oscillation is performed at a room temperature and at a threshold level of, for instance, 800 mA based upon distributed feedback. The oscillation can be observed to be a single axial mode in a spectrum of oscillation light wherein an oscillation wavelength is smoothly varied without any mode jumping.
Therefore, the DFB LD is highly expected to be utilized for light source for a high speed and long distance optical fiber communication and for an optical measurement instrument in a coherent optical system for the reason that single wavelength operation is advantageously performed therein, and the difference of oscillation threshold gains can be large between the main and sub-modes.
In the DFB LD described above, however, a fabricating yield is not obtained as high as expected in the actual fabricating process thereof. Further, a stabilized oscillation is not obtained therein as seen in such phenomena that, at the output level of more than 5 to 10 mW, a multi-axis oscillation occurs, and an axial mode jump results in spite of the construction in which .lambda./4 shift is actually performed in a diffraction grating as being apparent from the fact that an oscillation light spectrum inherent to a .lambda./4 shifted DFB LD is observed in an operation at a level less than an oscillation threshold.